How to Kill Algae Without Killing Fish

You can remove algae from your aquarium without harming your fish.
You can remove algae from your aquarium without harming your fish. (Image: aquarium image by Henryk Olszewski from

Whether you have a saltwater or freshwater aquarium, algae can be a problem. Although some algae in the aquarium can be beneficial to your fish, too much can cause the water to become polluted, clouding it. It can also cling to the glass, making everything look dirty, and making it difficult to see your fish. Algae can be tenacious, making it difficult to completely get rid of it. Using chemicals is often recommended, but these additives can be harmful to your fish. However, you can take several steps to safely kill the algae.

Things You'll Need

  • Water testing kit
  • Aeration system
  • Filtration system
  • Ultraviolet sterilization system

Clean the aquarium completely, wiping every surface. Perform partial water changes, exchanging dirty water from the aquarium with fresh water. These two steps will not only reduce the amount of algae in the aquarium, but will help to kill the algae strain by removing excess nutrients in the aquarium, such as organic waste particles, which the algae require to live.

Balance the nutrient levels in the tank. Nutrients, such as nitrites and phosphates, can build up in the tank. Algae require high levels of these nutrients to live. Use a water testing kit, which can be purchased at an aquarium supply store, to find out if the nutrient levels are high. Lower the nutrient levels either by adding stronger filtration, or by adding live plants to the aquarium. The plants will use the extra nutrients and out-compete the algae. Lowering these levels, if they are high, will not harm your fish, but will help to kill the algae.

Introduce stronger aeration or filtration to your aquarium. Some algae cannot thrive in an environment where the water is constantly in motion. By adding extra filtration or aeration, you create a stronger current, which may aid in killing the algae. In addition, extra filtration will help to reduce the nutrient levels, which algae require.

Introduce an ultraviolet sterilization system to your aquarium by connecting it to your lighting and filtration system. These specialized systems are designed to kill algae and bacteria in the aquarium. Ultraviolet sterilization systems can be found in aquarium supply stores.

Cover your aquarium or keep it in a darkened room for a period of days. This method of algae control is called a blackout period. Most types of algae need strong sunlight to live. Keeping the aquarium dim or dark will kill the algae.

Create a light schedule for your aquarium. Again, most algae need a lot of light to thrive. Keeping the lights on for only eight hours a day will help kill the algae in your aquarium.

Tips & Warnings

  • Filter any water thoroughly before you add it to the aquarium, to cut down on any chemicals that might be found in your tap water.

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